Micro device transfer head heater assembly and method of transferring a micro device

ABSTRACT

A method of transferring a micro device and an array of micro devices are disclosed. A carrier substrate carrying a micro device connected to a bonding layer is heated to a temperature below a liquidus temperature of the bonding layer, and a transfer head is heated to a temperature above the liquidus temperature of the bonding layer. Upon contacting the micro device with the transfer head, the heat from the transfer head transfers into the bonding layer to at least partially melt the bonding layer. A voltage applied to the transfer head creates a grip force which picks up the micro device from the carrier substrate.

RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.13/372,422 filed on Feb. 13, 2012, and claims the benefit of priorityfrom U.S. Provisional Patent Application Ser. No. 61/561,706 filed onNov. 18, 2011, U.S. Provisional Patent Application Ser. No. 61/594,919filed on Feb. 3, 2012, U.S. Provisional Patent Application Ser. No.61/597,109 filed on Feb. 9, 2012 and U.S. Provisional Patent ApplicationSer. No. 61/597,658 filed on Feb. 10, 2012, the full disclosures ofwhich are incorporated herein by reference.

BACKGROUND

1. Field

The present invention relates to micro devices. More particularlyembodiments of the present invention relate to a method of transferringone or more micro devices to a receiving substrate with a micro devicetransfer head.

2. Background Information

Integration and packaging issues are one of the main obstacles for thecommercialization of micro devices such as radio frequency (RF)microelectromechanical systems (MEMS) microswitches, light-emittingdiode (LED) display systems, and MEMS or quartz-based oscillators.

Traditional technologies for transferring of devices include transfer bywafer bonding from a transfer wafer to a receiving wafer. One suchimplementation is “direct printing” involving one bonding step of anarray of devices from a transfer wafer to a receiving wafer, followed byremoval of the transfer wafer. Another such implementation is “transferprinting” involving two bonding/de-bonding steps. In transfer printing atransfer wafer may pick up an array of devices from a donor wafer, andthen bond the array of devices to a receiving wafer, followed by removalof the transfer wafer.

Some printing process variations have been developed where a device canbe selectively bonded and de-bonded during the transfer process. In bothtraditional and variations of the direct printing and transfer printingtechnologies, the transfer wafer is de-bonded from a device afterbonding the device to the receiving wafer. In addition, the entiretransfer wafer with the array of devices is involved in the transferprocess.

SUMMARY OF THE INVENTION

A micro device transfer head and head array, and a method oftransferring one or more micro devices to a receiving substrate aredisclosed. For example, the receiving substrate may be, but is notlimited to, a display substrate, a lighting substrate, a substrate withfunctional devices such as transistors or integrated circuits (ICs), ora substrate with metal redistribution lines.

In an embodiment, a micro device transfer head includes a basesubstrate, a mesa structure including sidewalls, at least one iselectrode formed over the mesa structure, and a dielectric layercovering the electrode. For example, the micro device transfer head canincorporate a monopolar or bipolar electrode structure. The mesastructure can be separately or integrally formed with the basesubstrate. The sidewalls can be tapered and protrude away from the basesubstrate to a top surface of the mesa structure, with the electrodeformed on the top surface. An electrode lead may extend from theelectrode in order to make contact with wiring in the base substrate andconnect the micro device transfer head to the working electronics of anelectrostatic gripper assembly. The electrode leads can run from theelectrode on the top surface of the mesa structure and along a sidewallof the mesa structure. The electrode lead can alternatively rununderneath the mesa structure and connect to a via running through themesa structure to the electrode.

The electrode and electrode leads may be covered with a depositeddielectric layer. Suitable materials for the dielectric layer include,but are not limited to, aluminum oxide (Al₂O₃) and tantalum oxide(Ta₂O₅). Since the dielectric layer is deposited, the electrode andelectrode leads may be formed of a material which can withstand highdeposition temperatures, including high melting temperature metals suchas platinum and refractory metals or refractory metal alloys such astitanium tungsten (TiW).

In an embodiment, a method of transferring a micro device includespositioning a transfer head over a micro device connected to a carriersubstrate. The micro device is contacted with the transfer head and avoltage is applied to an electrode in the transfer head to create a grippressure on the micro device. The transfer head picks up the microdevice and then releases the micro device onto a receiving substrate.The voltage can be applied to the electrode prior to, while or aftercontacting the micro device with the transfer head. The voltage can be aconstant current voltage, or alternating current voltage. In anembodiment, an alternating current voltage is applied to a bipolarelectrode structure. In an embodiment, an operation is additionallyperformed to create a phase change in a bonding layer connecting themicro device to the carrier substrate prior to or while picking up themicro device.

In an embodiment, the bonding layer is heated to create a phase changefrom solid to liquid in the bonding layer prior to or while picking upthe micro device. Depending upon the operating conditions, a substantialportion of the bonding layer can be picked up and transferred with themicro device. A variety of operations can be performed to control thephase of the portion of the bonding layer when picking up, transferring,contacting the receiving substrate, and releasing the micro device andportion of the bonding layer on the receiving substrate. For example,the portion of the bonding layer which is picked up with the microdevice can be maintained in the liquid state when contacting thereceiving substrate and during the release operation onto the receivingsubstrate. In another embodiment, the portion of the bonding layer canbe allowed to cool to a solid phase after being pick up. For example,the portion of the bonding layer can be in a solid phase prior to orduring contacting the receiving substrate, and again melted to theliquid state during the release operation. A variety of temperature andmaterial phase cycles can be performed in accordance with embodiments ofthe invention.

In an embodiment, a method of transferring an array of micro devicesincludes positioning an array of transfer heads over an array of microdevices. The array of micro devices is contacted with the array oftransfer heads, and a voltage is selectively applied to a portion of thearray of transfer heads. Selectively applying a voltage may includeapplying a voltage to all of the transfer heads in the array, or to aportion corresponding to less than all of the transfer heads in thearray. The corresponding portion of the array of micro devices is thenpicked up with the portion of the array of transfer heads, and theportion of the array of micro devices is selectively released onto atleast one receiving substrate. In an embodiment, the array of transferheads may be rubbed on the array of micro devices while making contactin order to dislodge any particles which may be present on thecontacting surface of either of the transfer heads or micro devices. Inan embodiment, a phase change is created in an array of laterallyseparate locations of the bonding layer connecting the array of microdevices to the carrier substrate prior to picking up the array of microdevices.

In an embodiment, a method of fabricating a micro device transfer headarray includes forming an array of mesa structures on a base substrate,with each mesa structure including sidewalls. A separate electrode isformed over each mesa structure, and a dielectric layer is depositedover the array of mesa structures and each electrode. In an embodiment,the dielectric layer is deposited with atomic layer deposition (ALD),and may be pin-hole free. The dielectric layer may include one ormultiple dielectric layers. A conformal passivation layer may optionallybe grown or deposited over the base substrate and the array of mesastructures prior to forming the separate electrode over eachcorresponding mesa structure. In an embodiment, a conductive groundplane is formed over the dielectric layer and surrounding each of themesa structures.

In an embodiment, a method of transferring a micro device includesheating a carrier substrate carrying a micro device connected to abonding layer to a temperature below a liquidus temperature of thebonding layer, and heating a transfer head to a temperature above theliquidus temperature of the bonding layer. The micro device is contactedwith the transfer head and heat transfers from the transfer head intothe bonding layer to at least partially melt the bonding layer. Avoltage is applied to the transfer head to create a grip pressure on themicro device, and the micro device is picked up with the transfer head.The micro device can then be placed in contact with and released onto areceiving substrate. The receiving substrate may be globally or locallyheated to assist with the transfer process.

In an embodiment, a method of transferring an array of micro devicesincludes heating a substrate carrying an array of micro devicesconnected to a plurality of locations of a bonding layer to atemperature below a liquidus temperature of the bonding layer, andheating an array of transfer heads to a temperature above the liquidustemperature of the bonding layer. The array of micro devices arecontacted with the array of transfer heads and heat is transferred fromthe array of transfer heads into the plurality of locations of thebonding layer to at least partially melt portions of the plurality oflocations of the bonding layer. A voltage is selectively applied to aportion of the array of transfer heads, and a corresponding portion ofthe array of micro devices is picked up with the portion of the array oftransfer heads. The portion of the array of micro devices can then beplaced in contact with and selectively released onto at least onereceiving substrate. The receiving substrate may be globally or locallyheated to assist with the transfer process.

In an embodiment, the micro device and array of micro devices are microLED devices, each including a micro p-n diode and a metallization layer,with the metallization layer between the micro p-n diode and a bondinglayer formed on a substrate. When picking up the micro LED device andarray of micro LED devices may include picking up the micro p-n diode,the metallization layer and a portion of the bonding layer. A conformaldielectric barrier layer may span sidewalls of the micro p-n diode and abottom surface of the micro p-n diode. The conformal dielectric barrierlayer may be cleaved below the bottom surface of the micro p-n diode.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graphical illustration showing the pressure required toovercome the force of surface tension to pick up a micro device ofvarious dimensions in accordance with an embodiment of the invention.

FIG. 2 is a graphical illustration of the relationship between surfacetension and increasing gap distance created during a pick up operationin accordance with an embodiment of the invention.

FIG. 3 is a graphical illustration of the relationship between viscousforce pressures and increasing gap distance created during a pick upoperation at various pull rates in accordance with an embodiment of theinvention.

FIG. 4 is a graphical illustration obtained by modeling analysis showingthe grip pressure exerted by a transfer head on a micro device as thetransfer head is withdrawn from the micro device in accordance with anembodiment of the invention.

FIG. 5 is a cross-sectional side view illustration of a monopolar microdevice transfer head in accordance with an embodiment of the invention.

FIG. 6 is an isometric view illustration of a monopolar micro devicetransfer head in accordance with an embodiment of the invention.

FIG. 7 is a cross-sectional side view illustration of a bipolar microdevice transfer head in accordance with an embodiment of the invention.

FIG. 8 is an isometric view illustration of a bipolar micro devicetransfer head in accordance with an embodiment of the invention.

FIGS. 9-10 are top view illustrations of bipolar micro device transferheads in accordance with an embodiment of the invention.

FIG. 11 is an isometric view illustration of a bipolar micro devicetransfer head including conductive vias in accordance with an embodimentof the invention.

FIG. 12 is an isometric view illustration of a bipolar micro devicetransfer head array in accordance with an embodiment of the invention.

FIG. 13 is an isometric view illustration of a bipolar micro devicetransfer head array including a conductive ground plane in accordancewith an embodiment of the invention.

FIG. 14 is a cross-sectional side view illustration of a bipolar microdevice transfer head array including a conductive ground plane inaccordance with an embodiment of the invention.

FIG. 15 is a flow chart illustrating a method of picking up andtransferring a micro device from a carrier substrate to a receivingsubstrate in accordance with an embodiment of the invention.

FIG. 16 is a schematic illustration of an alternating voltage appliedacross a bipolar electrode in accordance with an embodiment of theinvention.

FIG. 17 is a schematic illustration of a constant voltage applied acrossa bipolar electrode in accordance with an embodiment of the invention.

FIG. 18 is a schematic illustration of a constant voltage applied to amonopolar electrode in accordance with an embodiment of the invention.

FIG. 19 is a cross-sectional side view illustration of a variety ofmicro LED structures including contact openings with a smaller widththan the top surface of the micro p-n diode.

FIG. 20 is a cross-sectional side view illustration of a variety ofmicro LED structures including contact openings with a larger width thanthe top surface of the micro p-n diode.

FIG. 21 is a cross-sectional side view illustration of a variety ofmicro LED structures including contact openings with the same width asthe top surface of the micro p-n diode.

FIG. 22 is a cross-sectional side view illustration of a wicked upbonding layer in accordance with an embodiment of the invention.

FIGS. 23A-23B include top and cross-sectional side view illustrations ofa carrier wafer and array of micro LED devices in accordance withembodiments of the invention.

FIG. 24 is a flow chart illustrating a method of picking up andtransferring a micro device from a carrier substrate to a receivingsubstrate in accordance with an embodiment of the invention.

FIG. 25 is a flow chart illustrating a method of picking up andtransferring an array of micro devices from a carrier substrate to atleast one receiving substrate in accordance with an embodiment of theinvention.

FIG. 26 is a cross-sectional side view illustration of an array of microdevice transfer heads in contact with an array of micro LED devices inaccordance with an embodiment of the invention.

FIG. 27 is a cross-sectional side view illustration of an array of microdevice transfer heads in contact with an array of micro LED devices inaccordance with an embodiment of the invention.

FIG. 28 is a cross-sectional side view illustration of an array of microdevice transfer heads picking up an array of micro LED devices inaccordance with an embodiment of the invention.

FIG. 29 is a cross-sectional side view illustration of an array of microdevice transfer heads picking up a portion of an array of micro LEDdevices in accordance with an embodiment of the invention.

FIG. 30 is a cross-sectional side view illustration of an array of microdevice transfer heads with an array of micro LED devices positioned overa receiving substrate in accordance with an embodiment of the invention.

FIG. 31 is a cross-sectional side view illustration of a micro deviceselectively released onto a receiving substrate in accordance with anembodiment of the invention.

FIG. 32 is a flow chart illustrating a method of picking up andtransferring a micro device from a carrier substrate to a receivingsubstrate in accordance with an embodiment of the invention.

FIG. 33A is a cross-sectional side view illustration of an at leastpartially melted location of a laterally continuous bonding layer inaccordance with an embodiment of the invention.

FIG. 33B is a cross-sectional side view illustration of at leastpartially melted locations of a laterally continuous bonding layer inaccordance with an embodiment of the invention.

FIG. 34A is a cross-sectional side view illustration of an at leastpartially melted laterally separate location of a bonding layer inaccordance with an embodiment of the invention.

FIG. 34B is a cross-sectional side view illustration of at leastpartially melted laterally separate locations of a bonding layer inaccordance with an embodiment of the invention.

FIG. 35A is a cross-sectional side view illustration of an at leastpartially melted laterally separate location of a bonding layer on apost in accordance with an embodiment of the invention.

FIG. 35B is a cross-sectional side view illustration of at leastpartially melted laterally separate locations of a bonding layer onposts in accordance with an embodiment of the invention.

FIG. 36 is a flow chart illustrating a method of picking up andtransferring an array of micro devices from a carrier substrate to atleast one receiving substrate in accordance with an embodiment of theinvention.

FIG. 37 is a cross-sectional side view illustration of an array of microdevice transfer heads in contact with an array of micro LED devices inaccordance with an embodiment of the invention.

FIG. 38 is a cross-sectional side view illustration of an array of microdevice transfer heads picking up an array of micro LED devices inaccordance with an embodiment of the invention.

FIG. 39 is a side view illustration of an array of micro device transferheads with an array of micro LED devices positioned over a receivingsubstrate in accordance with an embodiment of the invention.

FIG. 40 is a side view illustration of an array of micro LED devicesselectively released onto a receiving substrate in accordance with anembodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the present invention describe a micro device transferhead and head array, and method of transferring a micro device and anarray of micro devices to a receiving substrate. For example, thereceiving substrate may be, but is not limited to, a display substrate,a lighting substrate, a substrate with functional devices such astransistors or integrated circuits (ICs), or a substrate with metalredistribution lines. In some embodiments, the micro devices and arrayof micro devices described herein may be any of the micro LED devicestructures illustrated in FIGS. 19-21, and those described in relatedU.S. Provisional Application No. 61/561,706 and U.S. ProvisionalApplication No. 61/594,919. While some embodiments of the presentinvention are described with specific regard to micro LEDs, it is to beappreciated that embodiments of the invention are not so limited andthat certain embodiments may also be applicable to other micro devicessuch as diodes, transistors, ICs, and MEMS.

In various embodiments, description is made with reference to figures.However, certain embodiments may be practiced without one or more ofthese specific details, or in combination with other known methods andconfigurations. In the following description, numerous specific detailsare set forth, such as specific configurations, dimensions andprocesses, etc., in order to provide a thorough understanding of thepresent invention. In other instances, well-known semiconductorprocesses and manufacturing techniques have not been described inparticular detail in order to not unnecessarily obscure the presentinvention. Reference throughout this specification to “one embodiment,”“an embodiment” or the like means that a particular feature, structure,configuration, or characteristic described in connection with theembodiment is included in at least one embodiment of the invention.Thus, the appearances of the phrase “in one embodiment,” “an embodiment”or the like in various places throughout this specification are notnecessarily referring to the same embodiment of the invention.Furthermore, the particular features, structures, configurations, orcharacteristics may be combined in any suitable manner in one or moreembodiments.

The terms “over”, “to”, “between” and “on” as used herein may refer to arelative position of one layer with respect to other layers. One layer“over” or “on” another layer or bonded “to” another layer may bedirectly in contact with the other layer or may have one or moreintervening layers. One layer “between” layers may be directly incontact with the layers or may have one or more intervening layers.

The terms “micro” device or “micro” LED structure as used herein mayrefer to the descriptive size of certain devices or structures inaccordance with embodiments of the invention. As used herein, the terms“micro” devices or structures are meant to refer to the scale of 1 to100 μm. However, it is to be appreciated that embodiments of the presentinvention are not necessarily so limited, and that certain aspects ofthe embodiments may be applicable to larger, and possibly smaller sizescales.

In one aspect, embodiments of the invention describe a manner for masstransfer of an array of pre-fabricated micro devices with an array oftransfer heads. For example, the pre-fabricated micro devices may have aspecific functionality such as, but not limited to, a LED forlight-emission, silicon IC for logic and memory, and gallium arsenide(GaAs) circuits for radio frequency (RF) communications. In someembodiments, arrays of micro LED devices which are poised for pick upare described as having a 10 μm by 10 μm pitch, or 5 μm by 5 μm pitch.At these densities a 6 inch substrate, for example, can accommodateapproximately 165 million micro LED devices with a 10 μm by 10 μm pitch,or approximately 660 million micro LED devices with a 5 μm by 5 μmpitch. A transfer tool including an array of transfer heads matching aninteger multiple of the pitch of the corresponding array of micro LEDdevices can be used to pick up and transfer the array of micro LEDdevices to a receiving substrate. In this manner, it is possible tointegrate and assemble micro LED devices into heterogeneously integratedsystems, including substrates of any size ranging from micro displays tolarge area displays, and at high transfer rates. For example, a 1 cm by1 cm array of micro device transfer heads can pick up and transfer morethan 100,000 micro devices, with larger arrays of micro device transferheads being capable of transferring more micro devices. Each transferhead in the array of transfer heads may also be independentlycontrollable, which enables selective pick up and release of the microdevices.

In one aspect, without being limited to a particular theory, embodimentsof the invention describe micro device transfer heads and head arrayswhich operate in accordance with principles of electrostatic grippers,using the attraction of opposite charges to pick up micro devices. Inaccordance with embodiments of the present invention, a pull-in voltageis applied to a micro device transfer head in order to generate a gripforce on a micro device and pick up the micro device. Grip force isproportional to charged plate area so is calculated as a pressure.According to ideal electrostatic theory, a non-conductive dielectriclayer between a monopolar electrode and a conductive substrate yields agrip pressure in Pascal (Pa) in equation (1) of:

P=[∈ _(o)/2][V∈ _(r) /d] ²  (1)

where ∈_(o)=8.85.10⁻¹², V=electrode-substrate voltage in volts (V),∈_(r)=dielectric constant, and d=dielectric thickness in meters (m).With a bipolar gripper using two grip electrodes the voltage (V) in theabove equation is half of the voltage between electrodes A and B,[V_(A)−V_(B)]/2. The substrate potential is centered at the averagepotential, [V_(A)=V_(B)]/2. This average is generally zero withV_(A)=[−V_(B)].

In another aspect, embodiments of the invention describe a bonding layerwhich can maintain a micro device on a carrier substrate during certainprocessing and handling operations, and upon undergoing a phase changeprovides a medium on which the micro device can be retained yet is alsoreadily releasable from during a pick up operation. For example, thebonding layer may be remeltable or reflowable such that the bondinglayer undergoes a phase change from solid to liquid state prior to orduring the pick up operation. In the liquid state the bonding layer mayretain the micro device in place on a carrier substrate while alsoproviding a medium from which the micro device is readily releasable.Without being limited to a particular theory, in determining the grippressure which is necessary to pick up the micro device from the carriersubstrate the grip pressure should exceed the forces holding the microdevice to the carrier substrate, which may include but are not limitedto, surface tension forces, capillary forces, viscous effects, elasticrestoration forces, van-der-Waals forces, stiction and gravity.

In accordance with embodiments of the invention, when the dimensions ofa micro device are reduced below a certain range, the surface tensionforces of the liquid bonding layer holding the micro device to thecarrier substrate may become dominant over other forces holding themicro device. FIG. 1 is a graphical illustration of one embodimentobtained by modeling analysis showing the pressure required to overcomethe force of surface tension to pick up a micro device of variousdimensions, assuming a liquid indium (In) bonding layer with a surfacetension of 560 mN/m at the melting temperature of 156.7° C. For example,referring to FIG. 1 an exemplary 10 μm by 10 μm wide micro device isretained on a carrier substrate with a surface tension pressure ofapproximately 2.2 atmospheres (atm) with an indium bonding layer havinga liquid surface tension of 560 mN/m at its melting temperature of156.7° C. This is significantly larger than the pressure due to gravity,which is approximately 1.8×10⁻⁶ atm for an exemplary 10 μm×10 μm wide×3μm tall piece of gallium nitride (GaN).

Surface tension pressures and viscous effects may also be dynamic duringthe pick up operation. FIG. 2 is a graphical illustration of oneembodiment obtained by modeling analysis showing the relationship ofsurface tension and increasing gap distance created during the pick upoperation of an exemplary 10 μm by 10 μm wide micro device retained on acarrier substrate with a molten indium (In) bonding layer. The gapdistance along the x-axis referred to in FIG. 2 is the distance betweenthe bottom of the micro device and the carrier substrate, and starts at2 μm corresponding to an un-molten thickness of the In bonding layer. Asillustrated in FIG. 2, a surface tension pressure of 2.2 atm along they-axis is initially overcome by the grip pressure at the beginning ofthe pick up operation. As the micro device is then lifted from thecarrier substrate, the surface tension rapidly falls, with the pressureleveling out as the micro device is lifted further away from the carriersubstrate.

FIG. 3 is a graphical illustration of one embodiment obtained bymodeling analysis showing the relationship of viscous force pressures(atm) and increasing gap distance (μm) created during a pick upoperation at various pull rates for an exemplary 10 μm by 10 μm microdevice retained on a carrier substrate with a molten indium (In) bondinglayer. The gap distance referred to in FIG. 3 is the distance betweenthe bottom of the micro device and the carrier substrate, and starts at2 μm corresponding to an un-molten thickness of the In bonding layer. Asillustrated, viscous force pressures are more apparent during fasterlift speeds such as 1,000 mm/s than for slower lift speeds such as 0.1mm/s. Yet, the pressures generated from the viscous effects using theexemplary lift speeds illustrated in FIG. 3 are significantly less thanthe surface tension pressure generated and illustrated in FIG. 2 whichsuggests that surface tension pressure is the dominant pressure whichmust be overcome by the grip pressure during the pick up operation.

If an air gap of size (g) is present between the dielectric layer of themicro device transfer head and a top conductive surface of the microdevice then the grip pressure in equation (2) is:

P=[∈ _(o)/2][V∈ _(r)/(d+∈ _(r) g)]²  (2)

It is contemplated that an air gap can be present due to a variety ofsources including, but not limited to, particulate contamination,warpage, and misalignment of either surface of the transfer head ormicro device, or the presence of an additional layer on the transferhead or micro device, such as a lip of a conformal dielectric barrierlayer around the top conductive surface of a micro device. In aembodiment, a lip of a conformal dielectric barrier layer may createboth an air gap where a contact opening is formed and increase theeffective thickness of the dielectric layer of the transfer head wherethe lip is present.

As seen from equations (1) and (2) above, lower voltages may be utilizedwhere no air gap is present between the micro device transfer head andmicro device to be picked up. However, when an air gap is present thispresents a series capacitance in which the air capacitance may competewith the dielectric layer capacitance. In order to compensate for thepossibility of an air capacitance between any of an array of microdevice transfer heads over a corresponding array of micro devices to bepicked up, a higher operating voltage, higher dielectric constant forthe dielectric material, or thinner dielectric material may be used tomaximize the electric field. However, use of a higher electric field haslimitations due to possible dielectric breakdown and arcing.

FIG. 4 is a graphical illustration of one embodiment obtained bymodeling analysis showing the grip pressure exerted by a transfer headon a micro device as the transfer head is withdrawn from the topconductive surface of the micro device, corresponding to an increasingair gap size. The different lines correspond to different Ta₂O₅dielectric layer thicknesses between 0.5 μm and 2.0 μm on the transferhead, with the electric field being kept constant. As illustrated, noappreciable effect on grip pressure is observed at these conditionsbelow air gap sizes of approximately 1 nm (0.001 μm), and even as highas 10 nm (0.01 μm) for some conditions. However, it is to be appreciatedthat the tolerable air gap can be increased or decreased by changing theconditions. Thus, in accordance with some embodiments of the invention acertain amount of air gap tolerance is possible during the pick upoperation and actual contact with the micro device transfer head and thetop conductive surface of the micro device may not be necessary.

Now assuming that the grip pressure required to pick up the micro devicefrom the carrier substrate should exceed the sum of pressures retainingthe micro device on the carrier substrate (as well as any pressurereduction due to air gap) it is possible to derive the interrelationshipof operating voltage, dielectric constant and dielectric thickness ofthe dielectric material in the micro device transfer head by solving thegrip pressure equations. For purposes of clarity, assuming that the airgap distance is zero, for a monopolar electrode this becomes:

sqrt(P*2/∈_(o))=V∈ _(r) /d  (3)

Exemplary ranges of calculated dielectric thickness values are providedin Table 1 for desired grip pressures of 2 atm (202650 Pa) and 20 atm(2026500 Pa) for Al₂O₃ and Ta₂O₅ dielectric materials between operatingvoltages between 25 V and 300 V in order to illustrate theinterdependence of grip pressure, voltage, dielectric constant anddielectric thickness in accordance with an embodiment of the invention.The dielectric constants provided are approximate, and it is understoodthat the values can vary depending upon manner of formation.

TABLE 1 Dielectric Dielectric constant, ε_(r) Dielectric thickness,Material Voltage (V) (Hz-MHz range) d (microns) Grip pressure = 2 atmAl₂O₃ 25 9.8 1.1 Al₂O₃ 100 9.8 4.6 Al₂O₃ 300 9.8 13.7 Ta₂O₅ 25 25 2.9Ta₂O₅ 100 25 11.7 Ta₂O₅ 300 25 35.0 Grip pressure = 20 atm Al₂O₃ 25 9.80.4 Al₂O₃ 100 9.8 1.4 Al₂O₃ 300 9.8 4.3 Ta₂O₅ 25 25 0.9 Ta₂O₅ 100 25 3.7Ta₂O₅ 300 25 11.1

Since the grip pressure is proportional to the inverse square of thedielectric thickness, the calculated dielectric thicknesses in Table 1represents the maximum thicknesses which can be formed to achieve thenecessary grip pressure with the set operating voltage. Thicknesseslower than those provided in Table 1 may result in higher grip pressuresat the set operating voltage, however lower thicknesses increase theapplied electric field across the dielectric layer which requires thatthe dielectric material possess a dielectric strength sufficient towithstand the applied electric field without shorting. It is to beappreciated that the grip pressure, voltage, dielectric constant anddielectric thickness values provided in Table 1 are exemplary in nature,and provided in order to provide a foundation for working ranges of themicro device transfer head in accordance with embodiments of theinvention. The relationship between grip pressure, voltage, dielectricconstant and dielectric thickness values provided in Table 1 has beenillustrated in accordance with ideal electrostatic theory, andembodiments of the invention are not limited by such.

Referring now to FIG. 5, a side view illustration is provided of amonopolar micro device transfer head and head array in accordance withan embodiment of the invention. As shown, each monopolar device transferhead 100 may include a base substrate 102, a mesa structure 104including a top surface 108 and sidewalls 106, an optional passivationlayer 110 formed over the mesa structure 104 and including a top surface109 and sidewalls 107, an electrode 116 formed over the mesa structure104 (and optional passivation layer 110) and a dielectric layer 120 witha top surface 121 covering the electrode 116. Base substrate 102 may beformed from a variety of materials such as silicon, ceramics andpolymers which are capable of providing structural support. In anembodiment, base substrate has a conductivity between 10³ and 10¹⁸ohm-cm. Base substrate 102 may additionally include wiring (not shown)to connect the micro device transfer heads 100 to the workingelectronics of an electrostatic gripper assembly.

Mesa structure 104 may be formed using suitable processing techniques,and may be formed from the same or different material than basesubstrate 102. In one embodiment, mesa structure 104 is integrallyformed with base substrate 104, for example by using lithographicpatterning and etching, or casting techniques. In an embodiment,anisotropic etching techniques can be utilized to form tapered sidewalls106 for mesa structure 104. In another embodiment, mesa structure 104may be deposited or grown, and patterned on top of the base substrate102. In an embodiment, mesa structure 104 is a patterned oxide layer,such as silicon dioxide, formed over a semiconductor substrate, such assilicon.

In one aspect, the mesa structures 104 generate a profile whichprotrudes away from the base substrate so as to provide a localizedcontact point to pick up a specific micro device during a pick upoperation. In an embodiment, mesa structures 104 have a height ofapproximately 1 μm to 5 μm, or more specifically approximately 2 μm.Specific dimensions of the mesa structures 104 may depend upon thespecific dimensions of the micro devices to be picked up, as well as thethickness of any layers formed over the mesa structures. In anembodiment, the height, width, and planarity of the array of mesastructures 104 on the base substrate 102 are uniform across the basesubstrate so that each micro device transfer head 100 is capable ofmaking contact with each corresponding micro device during the pick upoperation. In an embodiment, the width across the top surface 121 ofeach micro device transfer head is slightly larger, approximately thesame, or less than the width of the top surface of the each micro devicein the corresponding micro device array so that a transfer head does notinadvertently make contact with a micro device adjacent to the intendedcorresponding micro device during the pick up operation. As described infurther detail below, since additional layers 110, 112, 120 may beformed over the mesa structure 104, the width of the mesa structure mayaccount for the thickness of the overlying layers so that the widthacross the top surface 121 of each micro device transfer head isslightly larger, approximately the same, or less than the width of thetop surface of the each micro device in the corresponding micro devicearray.

Still referring to FIG. 5, mesa structure 104 has a top surface 108,which may be planar, and sidewalls 106. In an embodiment, sidewalls 106may be tapered up to 10 degrees, for example. Tapering the sidewalls 106may be beneficial in forming the electrodes 116 and electrode leads 114as described further below. A passivation layer 110 may then beoptionally deposited or grown over the base substrate 102 and array ofmesa structures 104. Passivation layer 110 can be deposited by a varietyof suitable techniques such as chemical vapor deposition (CVD),sputtering, or atomic layer deposition (ALD). In an embodiment,passivation layer 110 may be 0.5 μm-2.0 μm thick oxide such as, but notlimited to, silicon oxide (SiO₂), aluminum oxide (Al₂O₃) or tantalumoxide (Ta₂O₅).

A conductive layer 112 may then be deposited over the array of mesastructures 104 and optional passivation layer 110, and patterned to formelectrodes 116 and electrode leads 114. For example, a lift offtechnique can be utilized to form the electrodes 116 and electrode leads114 in which a resist layer is deposited and patterned over thesubstrate, followed by deposition of a metal layer, and lift off of theresist and portion of the metal layer on the resist leaving behind thedesired pattern. Alternatively, metal layer deposition followed bypatterning and etching can be performed to achieve the desired pattern.Electrode leads 114 may run from the electrode 116 over the top surface108 of a mesa structure 104 (and top surface 109 of optional passivationlayer 110) and along a sidewall 106 of the mesa structure 104 (and alonga sidewall 107 of optional passivation layer 110). Conductive layer 112used to form the electrodes 116 and electrode leads 114 may be a singlelayer or multiple layers. A variety of conductive materials includingmetals, metal alloys, refractory metals, and refractory metal alloys maybe employed to form conductive layer 112. In an embodiment, theconductive layer 112 has a thickness up to 5,000 angstroms (0.5 μm). Inan embodiment, the conductive layer 112 includes a high meltingtemperature metal such as platinum or a refractory metal or refractorymetal alloy. For example, conductive layer may include platinum,titanium, vanadium, chromium, zirconium, niobium, molybdenum, ruthenium,rhodium, hafnium, tantalum, tungsten, rhenium, osmium, iridium andalloys thereof. Refractory metals and refractory metal alloys generallyexhibit higher resistance to heat and wear than other metals. In anembodiment, conductive layer 112 is an approximately 500 angstrom (0.05μm) thick titanium tungsten (TiW) refractory metal alloy.

A dielectric layer 120 is then deposited over the electrodes 116 andother exposed layers on the base substrate 102. In an embodiment, thedielectric layer 120 has a suitable thickness and dielectric constantfor achieving the required grip pressure of the micro device transferhead 100, and sufficient dielectric strength to not break down at theoperating voltage. The dielectric layer may be a single layer ormultiple layers. In an embodiment, the dielectric layer is 0.5 μm-2.0 μmthick, though thickness may be more or less depending upon the specifictopography of the transfer head 100 and underlying mesa structure 104.Suitable dielectric materials may include, but are not limited to,aluminum oxide (Al₂O₃) and tantalum oxide (Ta₂O₅). Referring back toTable 1 above, embodiments of Al₂O₃ dielectric layers with appliedelectric fields (determined by dividing the voltage by dielectricthickness) of 22 V/μm to 71 V/μm and Ta₂O₅ dielectric layers withapplied electric fields of 9 V/μm to 28 V/μm were provided. Inaccordance with embodiments of the invention, the dielectric layer 120possesses a dielectric strength greater than the applied electric fieldso as to avoid shorting of the transfer head during operation.Dielectric layer 120 can be deposited by a variety of suitabletechniques such as chemical vapor deposition (CVD), atomic layerdeposition (ALD) and physical vapor deposition (PVD) such as sputtering.Dielectric layer 120 may additionally be annealed following deposition.In one embodiment, the dielectric layer 120 possesses a dielectricstrength of at least 400 V/μm. Such a high dielectric strength can allowfor the use of a thinner dielectric layer than the calculatedthicknesses provided in exemplary Table 1. Techniques such as ALD can beutilized to deposit uniform, conformal, dense, and/or pin-hole freedielectric layers with good dielectric strength. Multiple layers canalso be utilized to achieve such a pin-hole free dielectric layer 120.Multiple layers of different dielectric materials may also be utilizedto form dielectric layer 120. In an embodiment, the underlyingconductive layer 112 includes platinum or a refractory metal orrefractory metal alloy possessing a melting temperature above thedeposition temperature of the dielectric layer material(s) so as to notbe a limiting factor in selecting the deposition temperature of thedielectric layer. In an embodiment, following the deposition ofdielectric layer 120 a thin coating (not illustrated) may be formed overthe dielectric layer 120 to provide a specific stiction coefficient toadd lateral friction and keep the micro devices from being knocked offthe transfer head during the pick up operation. In such an embodiment,the additional thin coating replaces top surface 121 as the contactingsurface, and this surface retains the dimensional array requirementsdescribed herein. Furthermore, the additional coating can affect thedielectric properties of the micro device transfer head which may affectthe operability of the micro device transfer head. In an embodiment, theadditional coating thickness can be minimal (e.g. below 10 nm) so as tohave little to no appreciable effect on the grip pressure.

FIG. 6 is a close-up isometric view of electrode 116 and electrode lead114 formed over an optional passivation layer 110 covering a mesastructure 104. For purposes of clarity, the overlying dielectric layer120 is not illustrated, and the optional passivation layer 110 and mesastructure 104 are illustrated as a single mesa structure/passivationlayer 104/110. In an exemplary embodiment, where the passivation layer110 and dielectric layer 120 are both 0.5 μm thick, the top surface108/109 of the mesa structure/passivation layer 104/110 onto which theelectrode 116 is formed is approximately 7 μm×7 μm in order to achieve a8 μm×8 μm top surface of the transfer head 100. In accordance with anembodiment, the electrode 116 covers the maximum amount of surface areaof the top surface 108/109 of the mesa structure/passivation layer104/110 as possible while remaining within patterning tolerances.Minimizing the amount of free space increases the capacitance, andresultant grip pressure which can be achieved by the micro devicetransfer head. While a certain amount of free space is illustrated onthe top surface 108/109 of the mesa structure/passivation layer 104/110in FIG. 6, the electrode 116 may cover the entire top surface 108/109.The electrode 116 may also be slightly larger than the top surface108/109, and partially extend down the sidewalls 106/107 of the mesastructure/passivation layer 104/110 to ensure complete coverage of thetop surface 108/109. It is to be appreciated that the mesa array mayhave a variety of different pitches, and that embodiments of theinvention are not limited to the exemplary 7 μm×7 μm top surface of themesa structure/passivation layer 104/110 in a 10 μm pitch.

Referring now to FIG. 7, a side view illustration is provided of abipolar micro device transfer head 100 and head array in accordance withan embodiment of the invention. As shown, the bipolar device transferhead 100 may include a base substrate 102, a mesa structure 104including a top surface 108 and sidewalls 106, passivation layer 110including a top surface 109 and sidewalls 107, a pair of electrodes116A, 116B and electrode leads 114A, 114B formed over the mesa structure104, optional passivation layer 110 and a dielectric layer 120 coveringthe pair of electrodes 116A, 116B.

FIG. 8 is a close-up isometric view of electrodes 116A, 116B andelectrode leads 114A, 114B formed over an optional passivation layer 110covering a mesa structure 104. For purposes of clarity, the overlyingdielectric layer 120 is not illustrated, and the optional passivationlayer 110 and mesa structure 104 are illustrated as a single mesastructure/passivation layer 104/110. FIG. 8 differs slightly from FIG. 7in that the electrode leads 114A, 114B are illustrated as running alonga single sidewall rather than on opposite sidewalls of the mesastructure/passivation layer 104/110. Electrode leads 114A, 114B may runalong any suitable sidewall in accordance with embodiments of theinvention. In an exemplary embodiment, where the top surface 108/109 ofthe mesa structure/passivation layer 104/110 is approximately 7 μm×7 μmcorresponding to a mesa array with a 10 μm pitch the electrodes maycover the maximum amount of the surface area of the top surface 108/109of the mesa structure/passivation layer 104/110 as possible while stillproviding separation between electrodes 116A, 116B. The minimum amountof separation distance may be balanced by considerations for maximizingsurface area, while avoiding overlapping electric fields from theelectrodes. For example, the electrodes 116A, 116B may be separated by0.5 μm or less, and the minimum separation distance may be limited bythe height of the electrodes. In an embodiment, the electrodes areslightly longer than the top surface 108/109 in one direction, andpartially extend down the sidewalls of the mesa structure/passivationlayer 104/110 to ensure maximum coverage of the top surface 108/109. Itis to be appreciated that the mesa array may have a variety of differentpitches, and that embodiments of the invention are not limited to theexemplary 7 μm×7 μm top surface of the mesa structure/passivation layer104/110 in a 10 μm pitch.

Referring now to FIGS. 9-10, top view illustrations of electrodes 116A,116B of a bipolar micro device transfer head are provided in accordancewith embodiments of the invention. Thus far, mesa structure 104 has beendescribed as a single mesa structure as shown in FIG. 9. However,embodiments of the invention are not so limited. In the embodimentillustrated in FIG. 10, each electrode 116 is formed on a separate mesastructure 104A, 104B separated by a trench 105. An optional passivationlayer 110 (not illustrated) can cover both mesa structures 104A, 104B.

Referring now to FIG. 11, an isometric view illustration of analternative electrode lead configuration is provided in accordance withan embodiment of the invention. In such an embodiment the electrodeleads 114A, 114B run underneath a portion of the mesa structure 104, andconductive vias 117A, 117B run through the mesa structure 104 (andoptional passivation layer 110 not illustrated) connecting theelectrodes 116A, 116B to the respective electrode leads 114A, 114B. Insuch an embodiment, electrode leads 114A, 114B may be formed prior toformation of mesa structure 104, and may be formed of the same ordifferent conductive material as electrode leads 114A, 114B andelectrodes 116A, 116B. While vias 117A, 117B are illustrated with regardto a bipolar electrode structure in FIG. 11 it is to be appreciated thatthe above described via or vias may also be integrated into monopolarelectrode structures.

Referring now to FIGS. 12-14, an embodiment of the invention isillustrated in which a conductive ground plane is formed over thedielectric layer and surrounding the array of mesa structures. FIG. 12is an isometric view illustration of an array of micro device transferheads 100 with a bipolar electrode configuration as previously describedwith regard to FIG. 8. For purposes of clarity, the optional underlyingpassivation layer and overlying dielectric layer have not beenillustrated. Referring now to FIGS. 13-14, a conductive ground plane 130is formed over the dielectric layer 120 and surrounding the array ofmesa structures 104. The presence of ground plane 130 may assist in theprevention of arcing between transfer heads 100, particularly during theapplication of high voltages. Ground plane 130 may be formed of aconductive material which may be the same as, or different as theconductive material used to form the electrodes, or vias. Ground plane130 may also be formed of a conductive material having a lower meltingtemperature than the conductive material used to form the electrodessince it is not necessary to deposit a dielectric layer of comparablequality (e.g. dielectric strength) to dielectric layer 120 after theformation of ground plane 130.

FIG. 15 is a flow chart illustrating a method of picking up andtransferring a micro device from a carrier substrate to a receivingsubstrate in accordance with an embodiment of the invention. Atoperation 1510 a transfer head is positioned over a micro deviceconnected to a carrier substrate. The transfer head may comprise a mesastructure, an electrode over the mesa structure, and a dielectric layercovering the electrode as described in the above embodiments. Thus, thetransfer head may have a monopolor or bipolar electrode configuration,as well as any other structural variations as described in the aboveembodiments. The micro device is then contacted with the transfer headat operation 1520. In an embodiment, the micro device is contacted withthe dielectric layer 120 of the transfer head. In an alternativeembodiment, the transfer head is positioned over the micro device with asuitable air gap separating them which does not significantly affect thegrip pressure, for example, 1 nm (0.001 μm) or 10 nm (0.01 μm). Atoperation 1530 a voltage is applied to the electrode to create a grippressure on the micro device, and the micro device is picked up with thetransfer head at operation 1540. The micro device is then released ontoa receiving substrate at operation 1550.

While operations 1510-1550 have been illustrated sequentially in FIG.15, it is to be appreciated that embodiments are not so limited and thatadditional operations may be performed and certain operations may beperformed in a different sequence. For example, in one embodiment, aftercontacting the micro device with the transfer head, the transfer head isrubbed across a top surface of the micro device in order to dislodge anyparticles which may be present on the contacting surface of either ofthe transfer head or micro device. In another embodiment, an operationis performed to create a phase change in the bonding layer connectingthe micro device to the carrier substrate prior to or while picking upthe micro device. If a portion of the bonding layer is picked up withthe micro device, additional operations can be performed to control thephase of the portion of the bonding layer during subsequent processing.

Operation 1530 of applying the voltage to the electrode to create a grippressure on the micro device can be performed in various orders. Forexample, the voltage can be applied prior to contacting the micro devicewith the transfer head, while contacting the micro device with thetransfer head, or after contacting the micro device with the transferhead. The voltage may also be applied prior to, while, or after creatingthe phase change in the bonding layer.

FIG. 16 is a schematic illustration of an alternating voltage appliedacross a bipolar electrode with the transfer head in contact with amicro device in accordance with an embodiment of the invention. Asillustrated, a separate alternating current (AC) voltage source may beapplied to each electrode lead 114A, 114B with an alternating voltageapplied across the pair of electrodes 116A, 116B so that at a particularpoint in time when a negative voltage is applied to electrode 116A, apositive voltage is applied to electrode 116B, and vice versa. Releasingthe micro device from the transfer head may be accomplished with avariety of methods including turning off the voltage sources, loweringthe voltage across the pair of electrodes, changing a waveform of the ACvoltage, and grounding the voltage source. FIG. 17 is a schematicillustration of a constant voltage applied to a bipolar electrode inaccordance with an embodiment of the invention. In the particularembodiment illustrated, a negative voltage is applied to electrode 116Awhile a positive voltage is applied to electrode 116B. FIG. 18 is aschematic illustration of a constant voltage applied to a monopolarelectrode in accordance with an embodiment of the invention. Once thetransfer head picks up the micro device illustrated in FIG. 18, theamount of time the transfer head can hold the micro device may be afunction of the discharge rate of the dielectric layer since only singlevoltage is applied to electrode 116. Releasing the micro device from thetransfer head illustrated in FIG. 14 may be accomplished by turning offthe voltage source, grounding the voltage source, or reversing thepolarity of the constant voltage.

In the particular embodiments illustrated in FIGS. 16-18, the microdevices 200 are those illustrated in FIG. 19, Example 19O. Though themicro devices illustrated in FIGS. 16-18 may be from any of the microLED device structures illustrated in FIGS. 19-21, and those described inrelated U.S. Provisional Application No. 61/561,706 and U.S. ProvisionalApplication No. 61/594,919. For example, a micro LED device 200 mayinclude a micro p-n diode 235, 250 and a metallization layer 220, withthe metallization layer between the micro p-n diode 235, 250 and abonding layer 210 formed on a substrate 201. In an embodiment, the microp-n diode 250 includes a top n-doped layer 214, one or more quantum welllayers 216, and a lower p-doped layer 218. The micro p-n diodes can befabricated with straight sidewalls or tapered sidewalls. In certainembodiments, the micro p-n diodes 250 possess outwardly taperedsidewalls 253 (from top to bottom). In certain embodiments, the microp-n diodes 235 possess inwardly tapered sidewalls 253 (from top tobottom). The metallization layer 220 may include one or more layers. Forexample, the metallization layer 220 may include an electrode layer anda barrier layer between the electrode layer and the bonding layer. Themicro p-n diode and metallization layer may each have a top surface, abottom surface and sidewalls. In an embodiment, the bottom surface 251of the micro p-n diode 250 is wider than the top surface 252 of themicro p-n diode, and the sidewalls 253 are tapered outwardly from top tobottom. The top surface of the micro p-n diode 235 may be wider than thebottom surface of the p-n diode, or approximately the same width. In anembodiment, the bottom surface 251 of the micro p-n diode 250 is widerthan the top surface 221 of the metallization layer 220. The bottomsurface of the micro p-n diode may also be wider than the top surface ofthe metallization layer, or approximately the same width as the topsurface of the metallization layer.

A conformal dielectric barrier layer 260 may optionally be formed overthe micro p-n diode 235, 250 and other exposed surfaces. The conformaldielectric barrier layer 260 may be thinner than the micro p-n diode235, 250, metallization layer 220 and optionally the bonding layer 210so that the conformal dielectric barrier layer 260 forms an outline ofthe topography it is formed on. In an embodiment, the micro p-n diode235, 250 is several microns thick, such as 3 μm, the metallization layer220 is 0.1 μm-2 μm thick, and the bonding layer 210 is 0.1 μm-2 μmthick. In an embodiment, the conformal dielectric barrier layer 260 isapproximately 50-600 angstroms thick aluminum oxide (Al₂O₃). Conformaldielectric barrier layer 260 may be deposited by a variety of suitabletechniques such as, but not limited to, atomic layer deposition (ALD).The conformal dielectric barrier layer 260 may protect against chargearcing between adjacent micro p-n diodes during the pick up process, andthereby protect against adjacent micro p-n diodes from sticking togetherduring the pick up process. The conformal dielectric barrier layer 260may also protect the sidewalls 253, quantum well layer 216 and bottomsurface 251, of the micro p-n diodes from contamination which couldaffect the integrity of the micro p-n diodes. For example, the conformaldielectric barrier layer 260 can function as a physical barrier towicking of the bonding layer material 210 up the sidewalls and quantumlayer 216 of the micro p-n diodes 150. The conformal dielectric barrierlayer 260 may also insulate the micro p-n diodes 250 once placed on areceiving substrate. In an embodiment, the conformal dielectric barrierlayer 260 span sidewalls 253 of the micro p-n diode, and may cover aquantum well layer 216 in the micro p-n diode. The conformal dielectricbarrier layer may also partially span the bottom surface 251 of themicro p-n diode, as well as span sidewalls of the metallization layer220. In some embodiments, the conformal dielectric barrier layer alsospans sidewalls of a patterned bonding layer 210. A contact opening 262may be formed in the conformal dielectric barrier layer 260 exposing thetop surface 252 of the micro p-n diode. In an embodiment, conformaldielectric barrier layer 260 is formed of the same material asdielectric layer 120 of the bonding head. Depending upon the particularmicro LED device structure, the conformal dielectric barrier layer 260may also span sidewalls of the bonding layer 210, as well as the carriersubstrate and posts, if present.

Referring to FIG. 19, the contact opening 262 may have a smaller widththan the top surface 252 of the micro p-n diode and the conformaldielectric barrier layer 260 forms a lip around the edges of the topsurface 252 of the micro p-n diode. Referring to FIG. 20, the contactopening 262 may have a slightly larger width than the top surface of themicro p-n diode. In such an embodiment, the contact opening 302 exposesthe top surface 252 of the micro p-n diode and an upper portion of thesidewalls 253 of the micro p-n diode, while the conformal dielectricbarrier layer 260 covers and insulates the quantum well layer(s) 216.Referring to FIG. 21, the conformal dielectric layer 260 may haveapproximately the same width as the top surface of the micro p-n diode.The conformal dielectric layer 260 may also span along a bottom surface251 of the micro p-n diodes illustrated in FIGS. 19-21.

Bonding layer 210 may be formed from a material which can maintain themicro LED device 200 on the carrier substrate 201 during certainprocessing and handling operations, and upon undergoing a phase changeprovide a medium on which the micro LED device 200 can be retained yetalso be readily releasable from during a pick up operation. For example,the bonding layer may be remeltable or reflowable such that the bondinglayer undergoes a phase change from solid to liquid state prior to orduring the pick up operation. In the liquid state the bonding layer mayretain the micro LED device in place on the carrier substrate while alsoproviding a medium from which the micro LED device 200 is readilyreleasable. In an embodiment, the bonding layer 210 has a liquidustemperature or melting temperature below approximately 350° C., or morespecifically below approximately 200° C. At such temperatures thebonding layer may undergo a phase change without substantially affectingthe other components of the micro LED device. For example, the bondinglayer may be formed of a metal or metal alloy, or a thermoplasticpolymer which is removable. For example, the bonding layer may includeindium, tin or a thermoplastic polymer such as polyethylene orpolypropylene. In an embodiment, the bonding layer may be conductive.For example, where the bonding layer undergoes a phase change from solidto liquid in response to a change in temperature a portion of thebonding layer may remain on the micro LED device during the pick upoperation. In such an embodiment, it may be beneficial that the bondinglayer is formed of a conductive material so that it does not adverselyaffect the micro LED device when it is subsequently transferred to areceiving substrate. In this case, the portion of conductive bondinglayer remaining on the micro LED device during the transfer may aid inbonding the micro LED device to a conductive pad on a receivingsubstrate. In a specific embodiment, the bonding layer may be formed ofindium, which has a melting temperature of 156.7° C. The bonding layermay be laterally continuous across the substrate 201, or may also beformed in laterally separate locations. For example, a laterallyseparate location of the bonding layer may have a width which is lessthan or approximately the same width as the bottom surface of the microp-n diode or metallization layer. In some embodiments, the micro p-ndiodes may optionally be formed on posts 202 on the substrate.

Solders may be suitable materials for bonding layer 210 since many aregenerally ductile materials in their solid state and exhibit favorablewetting with semiconductor and metal surfaces. A typical alloy melts nota single temperature, but over a temperature range. Thus, solder alloysare often characterized by a liquidus temperature corresponding to thelowest temperature at which the alloy remains liquid, and a solidustemperature corresponding to the highest temperature at which the alloyremains solid. An exemplary list of low melting solder materials whichmay be utilized with embodiments of the invention are provided in Table1.

TABLE 1 Liquidus Solidus Chemical composition Temperature (° C.)Temperature (° C.) 100 In 156.7 156.7 66.3In33.7Bi 72 7251In32.5Bi16.5Sn 60 60 57Bi26In17Sn 79 79 54.02Bi29.68In16.3Sn 81 8167Bi33In 109 109 50In50Sn 125 118 52Sn48In 131 118 58Bi42Sn 138 13897In3Ag 143 143 58Sn42In 145 118 99.3In0.7Ga 150 150 95In5Bi 150 12599.4In0.6Ga 152 152 99.6In0.4Ga 153 153 99.5In0.5Ga 154 154 60Sn40Bi 170138 100Sn 232 232 95Sn5Sb 240 235

An exemplary list thermoplastic polymers which may be utilized withembodiments of the invention are provided in Table 2.

TABLE 2 Polymer Melting Temperature (°C.) Acrylic (PMMA) 130-140Polyoxymethylene (POM or Acetal) 166 Polybutylene terephthalate (PBT)160 Polycaprolactone (PCL) 62 Polyethylene terephthalate (PET) 260Polycarbonate (PC) 267 Polyester 260 Polyethylene (PE) 105-130Polyetheretherketone (PEEK) 343 Polylactic acid (PLA) 50-80Polypropylene (PP) 160 Polystyrene (PS) 240 Polyvinylidene chloride(PVDC) 185

Referring now to FIG. 22, in accordance with some embodiments it ispossible that an amount of bonding layer wicked up along the sidesurfaces of the metallization layer 220 and along the bottom surface 251of the micro p-n diode 250 during fabrication of the array of micro p-ndiodes 250 on the carrier substrate 201. In this manner, conformaldielectric barrier layer 260 spanning along the bottom surface 251 ofthe micro p-n diodes 250 and side surfaces of the metallization layers220 may function as a physical barrier to protect the sidewalls 253 andquantum well layer 216 of the micro p-n diodes 250 from contamination bythe bonding layer material 210 during subsequent temperature cycles(particularly at temperatures above the liquidus or melting temperatureof the bonding layer material 210) such as during picking up the microLED devices from the carrier substrate, and releasing the micro LEDdevices onto the receiving substrate.

FIGS. 23A-23B include top and cross-sectional side view illustrations ofa carrier substrate 201 and array of micro LED devices in accordancewith an embodiment of the invention. In the particular embodimentsillustrated, the arrays are produced from micro LED devices of Example19N including micro p-n diode 150. However, it is to be appreciated thatFIGS. 23A-23B are meant to be exemplary, and that the array of micro LEDdevices can be formed from any of the micro LED devices previouslydescribed. In the embodiment illustrated in FIG. 23A, each individualmicro p-n diode 250 is illustrated as a pair of concentric circleshaving different diameters or widths corresponding the different widthsof the top and bottom surfaces of the micro p-n diode 250, and thecorresponding tapered sidewalls spanning between the top and bottomsurfaces. In the embodiment illustrated in FIG. 23B, each individualmicro p-n diode 250 is illustrated as a pair of concentric squares withtapered or rounded corners, with each square having a different widthcorresponding to the different widths of the top and bottom surfaces ofthe micro p-n diode 250, and the corresponding tapered sidewallsspanning from the top and bottom surfaces. However, embodiments of theinvention do not require tapered sidewalls, and the top and bottomsurfaces of the micro p-n diode 250 may have the same diameter, orwidth, and vertical sidewalls. As illustrated in FIGS. 23A-23B the arrayof micro LED devices is described as having a pitch (P), spacing (S)between each micro LED device and maximum width (W) of each micro LEDdevice. In order for clarity and conciseness, only x-dimensions areillustrated by the dotted lines in the top view illustration, though itis understood that similar y-dimensions may exist, and may have the sameor different dimensional values. In the particular embodimentsillustrated in FIGS. 23A-23B, the x- and y-dimensional values areidentical in the top view illustration. In one embodiment, the array ofmicro LED devices may have a pitch (P) of 10 μm, with each micro LEDdevice having a spacing (S) of 2 μm and maximum width (W) of 8 μm. Inanother embodiment, the array of micro LED devices may have a pitch (P)of 5 μm, with each micro LED device having a spacing (S) of 2 μm andmaximum width (W) of 3 μm. However, embodiments of the invention are notlimited to these specific dimensions, and any suitable dimension may beutilized.

FIG. 24 is a flow chart illustrating a method of picking up andtransferring a micro device from a carrier substrate to a receivingsubstrate in accordance with an embodiment of the invention. Atoperation 2410 a transfer head is positioned over a micro deviceconnected to a carrier substrate with a bonding layer. The transfer headmay be any transfer head described herein. The micro device may be anyof the micro LED device structures illustrated in FIGS. 19-21, and thosedescribed in related U.S. Provisional Application No. 61/561,706 andU.S. Provisional Application No. 61/594,919. The micro device is thencontacted with the transfer head at operation 2420. In an embodiment,the micro device is contacted with the dielectric layer 120 of thetransfer head. In an alternative embodiment, the transfer head ispositioned over the micro device with a suitable air gap separating themwhich does not significantly affect the grip pressure, for example, 1 nm(0.001 μm) or 10 nm (0.01 μm). At operation 2425 an operation isperformed to create a phase change in the bonding layer 210 from solidto liquid state. For example, the operation may include heating an Inbonding layer at or above the melting temperature of 156.7° C. Inanother embodiment, operation 2425 can be performed prior to operation2420. At operation 2430 a voltage is applied to the electrode to createa grip pressure on the micro device, and the micro device and asubstantial portion of the bonding layer 210 are picked up with thetransfer head at operation 2440. For example, approximately half of thebonding layer 210 may be picked up with the micro device. In analternative embodiment, none of the bonding layer 210 is picked up withthe transfer head. At operation 2445 the micro device and portion of thebonding layer 210 are placed in contact with a receiving substrate. Themicro device and portion of the bonding layer 210 are then released ontothe receiving substrate at operation 2450. A variety of operations canbe performed to control the phase of the portion of the bonding layerwhen picking up, transferring, contacting the receiving substrate, andreleasing the micro device and portion of the bonding layer 210 on thereceiving substrate. For example, the portion of the bonding layer whichis picked up with the micro device can be maintained in the liquid stateduring the contacting operation 2445 and during the release operation2450. In another embodiment, the portion of the bonding layer can beallowed to cool to a solid phase after being picked up. For example, theportion of the bonding layer can be in a solid phase during contactingoperation 2445, and again melted to the liquid state prior to or duringthe release operation 2450. A variety of temperature and material phasecycles can be performed in accordance with embodiments of the invention.

FIG. 25 is a flow chart illustrating a method of picking up andtransferring an array of micro devices from a carrier substrate to atleast one receiving substrate in accordance with an embodiment of theinvention. At operation 2510 an array of transfer heads is positionedover an array of micro devices, with each transfer head having a mesastructure, an electrode over the mesa structure, and a dielectric layercovering the electrode. At operation 2520 the array of micro devices arecontacted with the array of transfer heads. In an alternativeembodiment, the array of transfer heads is positioned over the array ofmicro devices with a suitable air gap separating them which does notsignificantly affect the grip pressure, for example, 1 nm (0.001 μm) or10 nm (0.01 μm). FIG. 26 is a side view illustration of an array ofmicro device transfer heads 100 in contact with an array of micro LEDdevices 200 in accordance with an embodiment of the invention. Asillustrated in FIG. 26, the pitch (P) of the array of transfer heads 100matches the pitch of the micro LED devices 200, with the pitch (P) ofthe array of transfer heads being the sum of the spacing (S) betweentransfer heads and width (W) of a transfer head.

In one embodiment, the array of micro LED devices 200 have a pitch of 10μm, with each micro LED device having a spacing of 2 μm and a maximumwidth of 8 μm. In an exemplary embodiment, assuming a micro p-n diode250 with straight sidewalls the top surface of the each micro LED device200 has a width of approximately 8 μm. In such an exemplary embodiment,the width of the top surface 121 of a corresponding transfer head 100 isapproximately 8 μm or smaller so as to avoid making inadvertent contactwith an adjacent micro LED device. In another embodiment, the array ofmicro LED devices 200 may have a pitch of 5 μm, with each micro LEDdevice having a spacing of 2 μm and a maximum width of 3 μm. In anexemplary embodiment, the top surface of the each micro LED device 200has a width of approximately 3 μm. In such an exemplary embodiment, thewidth of the top surface 121 of a corresponding transfer head 100 isapproximately 3 μm or smaller so as to avoid making inadvertent contactwith an adjacent micro LED device 200. However, embodiments of theinvention are not limited to these specific dimensions, and may be anysuitable dimension.

FIG. 27 is a side view illustration of an array of micro device transferheads in contact with an array of micro LED devices 200 in accordancewith an embodiment of the invention. In the embodiment illustrated inFIG. 27, the pitch (P) of the transfer heads is an integer multiple ofthe pitch of the array of micro devices. In the particular embodimentillustrated, the pitch (P) of the transfer heads is 3 times the pitch ofthe array of micro LED devices. In such an embodiment, having a largertransfer head pitch may protect against arcing between transfer heads.

Referring again to FIG. 25, at operation 2530 a voltage is selectivelyapplied to a portion of the array of transfer heads 100. Thus, eachtransfer head 100 may be independently operated. At operation 2540 acorresponding portion of the array of micro devices is picked up withthe portion of the array of transfer heads to which the voltage wasselectively applied. In one embodiment, selectively applying a voltageto a portion of the array of transfer heads means applying a voltage toevery transfer head in the array of transfer heads. FIG. 28 is a sideview illustration of every transfer head in an array of micro devicetransfer heads picking up an array of micro LED devices 200 inaccordance with an embodiment of the invention. In another embodiment,selectively applying a voltage to a portion of the array of transferheads means applying a voltage to less than every transfer head (e.g. asubset of transfer heads) in the array of transfer heads. FIG. 29 is aside view illustration of a subset of the array of micro device transferheads picking up a portion of an array of micro LED devices 200 inaccordance with an embodiment of the invention. In a particularembodiment illustrated in FIGS. 28-29, the pick up operation includespicking up the micro p-n diode 250, the metallization layer 220 and aportion of the conformal dielectric barrier layer 260 for the micro LEDdevice 200. In a particular embodiment illustrated in FIGS. 28-29, thepick up operation includes picking up a substantial portion of thebonding layer 210. Accordingly, any of the embodiments described withregard to FIGS. 25-31 may also be accompanied by controlling thetemperature of the portion of the bonding layer 210 as described withregard to FIG. 24. For example, embodiments described with regard toFIGS. 25-31 may include performing an operation to create a phase changefrom solid to liquid state in a plurality of locations of the bondinglayer connecting the array of micro devices to the carrier substrate 201prior to picking up the array of micro devices. In an embodiment, theplurality of locations of the bonding layer can be regions of the samebonding layer. In an embodiment, the plurality of locations of thebonding layer can be laterally separate locations of the bonding layer.

At operation 2550 the portion of the array of micro devices is thenreleased onto at least one receiving substrate. Thus, the array of microLEDs can all be released onto a single receiving substrate, orselectively released onto multiple substrates. For example, thereceiving substrate may be, but is not limited to, a display substrate,a lighting substrate, a substrate with functional devices such astransistors or ICs, or a substrate with metal redistribution lines.Release may be accomplished by affecting the applied voltage with any ofthe manners described with regard to FIGS. 16-18.

FIG. 30 is a side view illustration of an array of micro device transferheads holding a corresponding array of micro LED devices 200 over areceiving substrate 301 including a plurality of driver contacts 310.The array of micro LED devices 200 may then be placed into contact withthe receiving substrate and then selectively released. FIG. 31 is a sideview illustration of a single micro LED device 200 selectively releasedonto the receiving substrate 301 over a driver contact 310 in accordancewith an embodiment of the invention. In another embodiment, more thanone micro LED device 200 is released, or the entire array of micro LEDdevices 200 are released.

FIG. 32 is a flow chart illustrating a method of picking up andtransferring a micro device from a carrier substrate to a receivingsubstrate in accordance with an embodiment of the invention. For purposeof clarity, FIG. 32 is described in relation to various structuralconfigurations illustrated in FIGS. 33A-35B, though embodiments of theinvention are not so limited and may be practiced with other structuralconfigurations referred to herein. At operation 3210 a carrier substratecarrying a micro device connected to a bonding layer is optionallyheated to a temperature below a liquidus temperature of the bondinglayer. In an embodiment, the carrier substrate is heated to atemperature of 1° C. to 10° C. below a liquidus temperature of thebonding layer, though lower or higher temperatures may be used. The heatfrom the carrier substrate may transfer from the carrier substrate tothe bonding layer, to also maintain the bonding layer at approximatelythe same temperature. At operation 3220 a transfer head is heated to atemperature above the liquidus temperature of the bonding layer. Forexample, the transfer head may be heated to a temperature of 1° C. to150° C., and more specifically 1° C. to 50° C., above the liquidustemperature of the bonding layer, though higher temperatures may beused. The micro device is then contacted with the transfer head atoperation 3225, and heat is transferred from the transfer head 100 intothe bonding layer 210 to at least partially melt the bonding layer atoperation 3230. Alternatively, the micro device can be contacted withthe transfer head at operation 3225, followed by heating the transferhead to the temperature above the liquidus temperature of the bondinglayer at operation 3220 so that heat is transferred from the transferhead 100 into the bonding layer 210 to at least partially melt thebonding layer at operation 1030. Accordingly, it is to be understoodthat the order of operations in the flow charts illustrated in FIG. 32and FIG. 36 can be performed in different orders than the sequentiallynumbered operations. In an embodiment, the transfer head and carriersubstrate are heated to temperatures such that a sufficient portion ofthe bonding layer rapidly melts upon contacting the micro device withthe transfer head which is heated above the liquidus temperature so thatthe micro device may be picked up by the transfer head upon creating agrip force which overcomes the surface tension forces holding the microdevice to the carrier substrate. Size of the micro device, pick upspeed, and thermal conductivity of the system are factors in determiningthe temperatures.

FIG. 33A is a side view illustration of an at least partially meltedlocation 215 of a laterally continuous bonding layer directly below themicro LED device 200 in accordance with an embodiment of the invention.As illustrated, area 211 in location 215 of the bonding layer 210located directly below the micro device 200 is illustrated with a darkershading indicating that the area 211 is in the liquid state, while thelighter shaded portions 213 of bonding layer 210 are in the solid state.In the particular embodiment illustrated in FIG. 33A, the localizedmelting of area 211 of the bonding layer 210 may be accomplished byseparately heating the substrate 201 carrying the micro device 200, andthe transfer head assembly carrying the transfer head 100. For example,the carrier substrate 201 can be globally heated with an optionalheating element 402 (indicated by dotted lines) and heat distributionplate 400 to a temperature 1° C. to 10° C. below a liquidus temperatureof the bonding layer, and the transfer head can be heated with a heatingelement 502 and heat distribution plate 500 to a temperature of 1° C. to150° C., and more specifically 1° C. to 150° C., above the liquidustemperature of the bonding layer. Heat can be applied in other fashions,such as IR heat lamps, lasers, resistive heating elements, amongstothers. Substrate 201 may also be locally heated.

FIG. 33B is a side view illustration of at least partially meltedlocations of a laterally continuous bonding layer directly below themicro LED device 200 in accordance with an embodiment of the invention.As illustrated, the location of the bonding layer 210 located directlybelow the micro device 200 is illustrated with a darker shadingindicating that the area 211 is in the liquid state. In the particularembodiment illustrated in FIG. 33B, substantially all of the laterallycontinuous bonding layer 210 is in the liquid state 211, which may beaccomplished by globally heating the substrate 201 carrying the microdevice 200 to or above the liquidus temperature of the bonding layer210, for example with heating element 402 and heat distribution plate400, without requiring separate heating of the transfer head 100.

FIG. 34A is a side view illustration of an at least partially meltedlaterally separate location 215 of a bonding layer directly below themicro LED device 200 in accordance with another embodiment of theinvention. As illustrated, the locations 215 of the bonding layer 210directly below the micro devices 200 are laterally separate locations,with the laterally separate location 215 of the bonding layer locateddirectly below the micro device 200 which is in contact with thetransfer head 100 at least partially melted, indicated by shading ofarea 211. Similar to FIG. 33A, localized melting of area 211 of thelaterally separate location of bonding layer 210 may be accomplished byseparately heating the substrate 201 carrying the micro device 200, andthe transfer head assembly carrying the transfer head 100. Heatingelement 402 may be optional for localized heating, indicated by thedotted lines. Carrier substrate 201 may also be locally heated.

FIG. 34B is a side view illustration of at least partially meltedlaterally separate locations of a bonding layer in accordance with anembodiment of the invention. As illustrated, the laterally separatelocations 215 of the bonding layer 210 located below the micro devices200 are illustrated with a darker shading indicating that areas 211 arein the liquid state. In the particular embodiment illustrated in FIG.34B, substantially all of each laterally separate location 215 of thebonding layer 210 is molten, which may be accomplished by globallyheating the substrate 201 carrying the micro devices 200 to or above theliquidus temperature of the bonding layer 210, for example with heatingelement 402 and heat distribution plate 400, without requiring separateheating of the transfer head 100.

FIG. 35A is a side view illustration of an at least partially meltedlaterally separate location 215 of a bonding layer on a post 202 inaccordance with an embodiment of the invention. As illustrated, thelocations 215 of the bonding layer 210 located below the micro devices200 are laterally separate locations, with the laterally separatelocation 215 of the bonding layer located below the micro device 200 incontact with the transfer head 100 at least partially melted, indicatedby shading of area 211. Similar to FIG. 33A, localized melting of area211 of the laterally separate location 215 of bonding layer 210 may beaccomplished by separately heating the substrate 201 carrying the microdevice 200, and the transfer head assembly carrying the transfer head100. Heating element 402 may be optional for localized heating,indicated by the dotted lines. Carrier substrate 201 may also be locallyheated.

FIG. 35B is a side view illustration of at least partially meltedlaterally separate locations 215 of a bonding layer on posts 202 inaccordance with an embodiment of the invention. As illustrated, thelaterally separate locations of the bonding layer 210 located below themicro devices 200 are illustrated with a darker shading indicating thatareas 211 are in the liquid state. In the particular embodimentillustrated in FIG. 35B, each laterally separate location 215 of thebonding layer 210 is molten, which may be accomplished by globallyheating the substrate 201 carrying the micro devices 200 to or above theliquidus temperature of the bonding layer 210, for example with heatingelement 402 and heat distribution plate 400, without requiring separateheating of the transfer head 100.

Referring again to FIG. 32 a voltage is applied to the electrode(s) 116in the transfer head 100 to create a grip pressure on the micro device200 at operation 3240, and at operation 3250 the micro device is pickedup with the transfer head. As described above, the order of operationsin the flow charts illustrated in FIG. 32 and FIG. 36 can be performedin different orders than the sequentially numbered operations. Forexample, operation 3240 of applying a voltage to the transfer head tocreate a grip pressure on the micro device can be performed earlier inthe sequence of operations. In an embodiment, a substantial portion ofthe bonding layer 210 is picked up with the transfer head 100 atoperation 3245. For example, approximately half of the bonding layer 210may be picked up with the micro device 200. In an alternativeembodiment, none of the bonding layer 210 is picked up with the transferhead. In an embodiment, a portion of the conformal dielectric barrierlayer 260 is picked up with the micro device 200. For example, a portionof the conformal dielectric barrier layer spanning sidewalls 235 and aportion of the bottom surface 251 of the micro device is picked up withthe micro device. The portion of conformal dielectric barrier layerspanning the sidewalls 253 may cover a quantum well layer 216 of themicro device. At operation 1050 the micro device and optionally aportion of the bonding layer 210 and conformal dielectric barrier layer260 are placed in contact with a receiving substrate. The micro deviceand optionally a portion of the bonding layer 210 and conformaldielectric barrier layer 260 are then released onto the receivingsubstrate at operation 1060.

Referring again to FIGS. 33A-35B, in the particular embodimentillustrated the bottom surface of the micro p-n diode 250 is wider thanthe top surface of the metallization layer 220, and the conformaldielectric barrier layer 260 spans the sidewalls of the micro p-n diode250, a portion of the bottom surface of the micro p-n diode 150 andsidewalls of the metallization layer 220. In one aspect, the portion ofthe conformal dielectric barrier layer 260 wrapping underneath the microp-n diode 250 protects the conformal dielectric barrier layer 260 on thesidewalls of the micro p-n diode 250 from chipping or breaking duringthe pick up operation with the transfer head 100. Stress points may becreated in the conformal dielectric barrier layer 260 adjacent themetallization layer 220 or bonding layer 210, particularly at cornersand locations with sharp angles. Upon contacting the micro LED devicewith the transfer head 100 and/or creating the phase change in thebonding layer, these stress points become natural break points in theconformal dielectric barrier layer 260 at which the conformal dielectriclayer can be cleaved. In an embodiment, the conformal dielectric barrierlayer 260 is cleaved at the natural break points after contacting themicro LED device with the transfer head and/or after creating the phasechange in the bonding layer, which may be prior to or during picking upthe micro p-n diode and the metallization layer. In the liquid state thebonding layer 210 may smooth out over the underlying structure inresponse to compressive forces associated with contacting the micro LEDdevice with the transfer head 100. In an embodiment, after contactingthe micro LED device with the transfer head, the transfer head is rubbedacross a top surface of the micro LED device prior to creating the phasechange in the bonding layer. Rubbing may dislodge any particles whichmay be present on the contacting surface of either of the transfer heador micro LED device. Rubbing may also transfer pressure to the conformaldielectric barrier layer. Thus, both transferring a pressure from thetransfer head 100 to the conformal dielectric barrier layer 260 andheating the bonding layer above a liquidus temperature of the bondinglayer can contribute to cleaving the conformal dielectric barrier layer260 at a location underneath the micro p-n diode 250 and may preservethe integrity of the micro LED device and quantum well layer. In anembodiment, the bottom surface of the micro p-n diode 250 is wider thanthe top surface of the metallization layer 220 to the extent that thereis room for the conformal dielectric barrier layer 260 to be formed onthe bottom surface of the micro p-n diode 250 and create break points,though this distance may also be determined by lithographic tolerances.In an embodiment, a 0.25 μm to 1 μm distance on each side of the microp-n diode 250 accommodates a 50 angstrom to 600 angstrom thick conformaldielectric barrier layer 260.

A variety of operations can be performed to control the phase of theportion of the bonding layer when picking up, transferring, contactingthe receiving substrate, and releasing the micro device and portion ofthe bonding layer 210 on the receiving substrate. For example, theportion of the bonding layer which is picked up with the micro devicecan be maintained in the liquid state during the contacting operation3250 and during the release operation 3260. In another embodiment, theportion of the bonding layer can be allowed to cool to a solid phaseafter being picked up. For example, the portion of the bonding layer canbe in a solid phase during contacting operation 3250, and again meltedto the liquid state prior to or during the release operation 3260. Avariety of temperature and material phase cycles can be performed inaccordance with embodiments of the invention.

An exemplary embodiment which illustrates controlling the phase of theportion of the bonding layer when picking up, transferring, contactingthe receiving substrate, and releasing the micro device of FIG. 33A isdescribed in additional detail in the following method illustrated inFIG. 36 and the structural configurations illustrated in FIGS. 37-40,though embodiments of the invention are not so limited an may bepracticed with other structural configurations. At operation 3610 asubstrate carrying an array of micro devices connected to a plurality oflocations of a bonding layer is optionally heated to a temperature belowa liquidus temperature of the bonding layer. The heat from the carriersubstrate may transfer from the carrier substrate to the bonding layer,to also maintain the bonding layer at approximately the sametemperature. At operation 3620 a transfer head is heated to atemperature above the liquidus temperature of the bonding layer. Thearray of micro devices are then contacted with the array of transferheads at operation 3625, and heat is transferred from the array oftransfer heads 100 into the plurality of locations of the bonding layer210 to at least partially melt portions of the plurality of locations ofthe bonding layer at operation 3630. Alternatively, the array of microdevices can be contacted with the array of transfer heads at operation3625, followed by heating the array of transfer heads to the temperatureabove the liquidus temperature of the bonding layer at operation 3620 sothat heat is transferred from the array of transfer heads 100 into theplurality of locations of the bonding layer 210 to at least partiallymelt the portions of the plurality of locations of the bonding layer atoperation 3630. Accordingly, it is to be understood that the order ofoperations in the flow charts illustrated in FIG. 32 and FIG. 36 can beperformed in different orders than the sequentially numbered operations.

FIG. 37 is a side view illustration of an array of micro device transferheads in contact with an array of micro LED devices of FIG. 33A, inwhich the plurality of locations of the bonding layer are at leastpartially melted, indicated by the dark shaded areas 211, in accordancewith an embodiment of the invention. In the particular embodimentillustrated in FIG. 37, the localized melting of areas 211 of thebonding layer 210 may be accomplished by separately heating the carriersubstrate 201 carrying the micro devices 200, and the array of transferheads 100. For example, the carrier substrate 201 can be heated with aheating element 402 and heat distribution plate 400 to a temperature 1°C. to 10° C. below a liquidus temperature of the bonding layer, and thebase array of transfer heads 100 can be heated with a heating element502 and heat distribution plate 500 to a temperature of 1° C. to 150°C., and more specifically 1° C. to 150° C., above the liquidustemperature of the bonding layer as described in relation to FIG. 33A.Heat can be applied in other fashions, such as IR heat lamps, lasers,resistive heating elements, amongst others. Carrier substrate 201 mayalso be locally heated.

Referring again to FIG. 36 a voltage is then selectively applied to theelectrode(s) 116 in a portion of the array of transfer heads 100 tocreate a grip pressure on the corresponding array of micro devices 200at operation 3640, and at operation 3645 the corresponding portion ofthe array of micro devices 200 are picked up with the portion of thearray of transfer heads 100. As described above, the order of operationsin the flow charts illustrated in FIG. 32 and FIG. 36 can be performedin different orders than the sequentially numbered operations. Forexample, operation 3640 of applying a voltage to the transfer head tocreate a grip pressure on the micro device can be performed earlier inthe sequence of operations. In an embodiment, a substantial portion ofthe plurality of locations of the bonding layer 210 is picked up withthe array of micro devices 200 at operation 3645. For example,approximately half of the plurality of locations of the bonding layer210 may be picked up with the array of micro devices 200. In analternative embodiment, none of the bonding layer 210 is picked up withthe array of micro devices 200. In an embodiment, a portion of theconformal dielectric barrier layer 260 is picked up with the microdevices 200. For example, a portion of the conformal dielectric barrierlayer spanning sidewalls 235 and a portion of the bottom surfaces 251 ofthe micro devices is picked up with the micro devices. The portion ofconformal dielectric barrier layer spanning the sidewalls 253 may covera quantum well layer 216 in each of the micro devices. FIG. 38 is a sideview illustration of an array of micro device transfer heads 100 pickingup an array of micro LED devices 200 in accordance with an embodiment ofthe invention, in which a substantial portion of the plurality locationsof bonding layer are picked up in the liquid state 211 along with thearray of micro LED devices 200.

At operation 3650 the corresponding portion of the array of microdevices 200 and optionally the portion of the bonding layer 210 andportion of the conformal dielectric barrier layer 260 which have beenpicked up are placed in contact with a receiving substrate. The bondinglayer 210 may be in either the solid state 213 or liquid state 211 whencontacting the substrate. The portion of the array of micro devices andoptionally the portion of the bonding layer 210 and portion of theconformal dielectric barrier layer 260 are then selectively releasedonto the at least one receiving substrate at operation 3660. Thus, thearray of micro devices can all be released onto a single receivingsubstrate, or selectively released onto multiple substrates. Thereceiving substrate may be, but is not limited to, a display substrate,a lighting substrate, a substrate with functional devices such astransistors or ICs, or a substrate with metal redistribution lines.Release may be accomplished by turning off the voltage source, groundingthe voltage source, or reversing the polarity of the constant voltage.

FIG. 39 is a side view illustration of an array of micro device transferheads with an array of micro LED devices positioned over a receivingsubstrate 301 including a plurality of driver contacts 310 in accordancewith an embodiment of the invention, in which the portions of thebonding layer which have been picked up are in the liquid state 211.FIG. 40 is a side view illustration of an array of micro LED devicesselectively released onto the receiving substrate 301 over the drivercontacts 310 in accordance with an embodiment of the invention. Inanother embodiment, a single micro LED device 200 or a portion of themicro LED devices 200 are released. Upon release of the micro devices200 onto the receiving substrate 301 the corresponding portions of thebonding layer are allowed to cool to the solid state 213.

In an embodiment, the receiving substrate 301 can be heated to atemperature above or below the liquidus temperature of the bonding layer210 to assist with the transfer process. The receiving substrate 301 canalso be locally or globally heated. In one embodiment, the receivingsubstrate is globally heated with a heating element 602 and heatdistribution plate 600 similar to the carrier substrate. Heat can beapplied in other fashions, such as IR heat lamps, lasers, resistiveheating elements, amongst others. In one embodiment, a localized lasercan be provided above a top surface of the receiving substrate 301 toprovide localized heating to the bonding layer or receiving substrate.In another embodiment, a localized laser can be provided below a bottomsurface of the receiving substrate 301, so that the bonding layer orreceiving substrate is locally heated from the backside. Where localizedheating of the receiving substrate 301 is utilized, for example bylaser, temperatures below or above the liquidus temperature of thebonding layer may be accomplished. For example, a local region ofreceiving substrate 301 adjacent contact 310 can be locally heated to orabove the liquidus temperature of the bonding layer to facilitatebonding, followed by cooling to solidify the bond. Likewise, thereceiving substrate 301 can be locally or globally maintained at anelevated temperature below the liquidus temperature of the bondinglayer, or allowed to remain at room temperature.

A variety of operations can be performed to control the phase of theportion of the bonding layer when picking up, transferring, contactingthe receiving substrate, and releasing the micro devices and portion ofthe bonding layer 210 on the receiving substrate. For example, theportion of the bonding layer which is picked up with the micro devicecan be maintained in the liquid state during the contacting operation3650 and during the release operation 3660. In another embodiment, theportion of the bonding layer can be allowed to cool to a solid phaseafter being picked up. For example, the portion of the bonding layer canbe in a solid phase during contacting operation 3650, and again meltedto the liquid state prior to or during the release operation 3660. Avariety of temperature and material phase cycles can be performed inaccordance with embodiments of the invention.

In utilizing the various aspects of this invention, it would becomeapparent to one skilled in the art that combinations or variations ofthe above embodiments are possible for forming a micro device transferhead and head array, and for transferring a micro device and microdevice array. Although the present invention has been described inlanguage specific to structural features and/or methodological acts, itis to be understood that the invention defined in the appended claims isnot necessarily limited to the specific features or acts described. Thespecific features and acts disclosed are instead to be understood asparticularly graceful implementations of the claimed invention usefulfor illustrating the present invention.

What is claimed is:
 1. A transfer head assembly comprising: a heaterassembly; and a base substrate supporting an array of electrostatictransfer heads; wherein the heater assembly is configured to heat eachelectrostatic transfer head to a temperature above 156.7° C.
 2. Thetransfer head assembly of claim 1, wherein the heater assembly comprisesa heating element and a heat distribution plate.
 3. The transfer headassembly of claim 1, wherein each electrostatic transfer head comprisesa mesa structure and a dielectric layer covering the mesa structure. 4.The transfer head assembly of claim 1, wherein each electrostatictransfer head incorporates a monopolar electrode configuration.
 5. Thetransfer head assembly of claim 1, wherein each electrostatic transferhead incorporates a bipolar electrode configuration.
 6. The transferhead assembly of claim 1, wherein each mesa structure is integrallyformed with the base substrate.
 7. The transfer head assembly of claim1, wherein the dielectric layer is 0.5-2.0 μm thick.
 8. The transferhead assembly of claim 3, wherein each mesa structure is 1-5 μm thick.9. The transfer head assembly of claim 1, wherein each electrostatictransfer head is configured to apply a separate electrostatic grippressure to a separate micro device of 1 to 100 μm scale.
 10. A transfertool comprising: a transfer head assembly comprising: a heater assembly;and a base substrate supporting an array of electrostatic transferheads; wherein the heater assembly is configured to heat eachelectrostatic transfer head to a temperature above 156.7° C.; and acarrier substrate heater assembly.
 11. The transfer tool of claim 10,wherein the carrier substrate heater assembly is configured to heat acarrier substrate to 146.7° C.
 12. The transfer tool of claim 10,wherein the carrier substrate heater assembly is configured to heat acarrier substrate to 349° C.
 13. The transfer tool of claim 10, whereinthe carrier substrate heater assembly comprises a carrier substrateheating element and a carrier substrate heat distribution plate.
 14. Thetransfer tool of claim 10, further comprising a receiving substrateheater assembly.
 15. The transfer tool of claim 12, wherein thereceiving substrate heater assembly is configured to heat a receivingsubstrate to 146.7° C.
 16. The transfer tool of claim 12, wherein thereceiving substrate heater assembly is configured to heat a receivingsubstrate to 349° C.
 17. The transfer tool of claim 12, wherein thereceiving substrate heater assembly comprises a receiving substrateheating element and a receiving substrate heat distribution plate. 18.The transfer tool of claim 10, wherein each electrostatic transfer headincorporates a monopolar electrode configuration.
 19. The transfer toolof claim 10, wherein each electrostatic transfer head incorporates abipolar electrode configuration.
 20. The transfer tool of claim 10,wherein each electrostatic transfer head is configured to apply aseparate electrostatic grip pressure to a separate micro device of 1 to100 μm scale.